While liquid crystal displays (LCDs) offer a compact, lightweight alternative to cathode ray tube (CRT) monitors, there are many applications for which the image quality of LCD displays are not yet satisfactory, particularly as the relative size of these devices increases. Larger LCD panels, such as those used in laptop computer or larger displays, are transmissive, and thus require a backlight. This type of light-providing surface, positioned behind the LCD panel, directs light outwards and towards the LCD.
Conventional approaches for backlighting use various arrangements of cold cathode fluorescent (CCFL) light sources with light guide plates, one or more types of enhancement films, polarization films, reflective surfaces, and other light conditioning elements. Conventional flat panel backlight solutions using side-mounted CCFLs are less and less desirable as display size increases and, particularly as display area grows, can be susceptible to warping in manufacture or due to heat. Light-guiding backlight techniques that are conventionally employed for smaller devices are increasingly hampered by low brightness or luminance levels and by problems related to poor uniformity as the display size increases, such as would be needed for digital TV, for example. Existing backlight apparatus for LCD displays and other display and illumination applications, often using banks of CCFLs lined up in parallel, can be relatively inefficient. These display solutions can also be relatively thick, due to the need to house the CCFL and its supporting films and surfaces behind the LC panel. The CCFL light source itself presents an environmental problem for disposal, since these devices contain some amount of mercury. To compensate for uniformity and brightness problems with conventional CCFL-based backlights, a number of supporting films are conventionally interposed between the backlight and the display, or disposed following the display, such as relatively high-cost reflective polarization films for example. As is well known, the spectral characteristics of CCFLs are relatively poor when compared to other types of light sources.
Faced with the inherent difficulties and limitations to CCFL used in backlighting applications, researchers have been motivated to pursue alternative backlighting approaches. A number of solutions have been proposed utilizing Light-Emitting Diodes (LEDs). Recent advances in LED brightness, color output, and overall performance, with continuing reduction in cost, make LEDs, lasers, and solid-state light sources in general particularly attractive. However, because LEDs and lasers act as point light sources, appropriate solutions are needed for redirecting and spreading this light to provide the uniform plane of light that is needed for backlighting and to provide the necessary color uniformity.
One approach for providing backlight illumination using LEDs is using an array arrangement, such as that described in the paper by M. Zeiler, J. Huttner, L. Plotz, and H. Ott entitled “Late-News Paper: Optimization Parameters for LED Backlighting Solutions” SID 2006 Digest pp. 1524-1527. Using this type of solution, an array of LED clusters using Red (R), Green (G), and Blue (B) LEDs is deployed as a backlight for an LCD displays. Two types of clusters are described: RGGB and RGB. Similarly, U.S. Pat. No. 6,789,921 entitled “Method and Apparatus for Backlighting a Dual Mode Liquid Crystal Display” to Deloy et al. describes an array arrangement used for an instrument panel. However, except for specialized uses such as for some types of instrument panels and for very high-end monitors and TV panels, array arrangements do not appear promising, due to problems of poor color and brightness uniformity, high parts count, high heat, and dimensional requirements.
Light guides have been employed for spreading light from a point source in order to form a line of light. For example, U.S. Pat. No. 5,499,112 entitled “Light Guide, Illuminating Device Having the Light Guide, and Image Reading Device and Information Processing Apparatus Having the Illuminating Device” to Kawai et al. discloses redirecting light from one or more LEDs to a line in a scanning apparatus, using a single light guide having extraction features distributed along its length. U.S. Pat. No. 5,400,224 entitled “Lighting Panel” to DuNah et al. describes a molded panel assembly having multiple light guides that are treated with randomized roughness over a back surface for backlighting illumination.
A number of solutions have been proposed for redistributing LED light over a larger area, along a light guiding panel. One proposed solution is the MicroLens™ molded light guide from Global Lighting Technologies Inc., Brecksville, Ohio that spreads light from a single LED over a larger light panel. Similarly, U.S. Patent Application Publication No. 2003/0123246 entitled “Light Emitting Panel Assemblies” by Parker shows a small-scale light panel using multiple point sources with optical “deformities” that redirect light into the panel.
Another type of solution first directs the light from the LED, lamp, or other point source along a line, then spread this light over a panel. For example, U.S. Pat. No. 5,835,661 entitled “Light Expanding System for Producing a Linear or Planar Light Beam from a Point-Like Light Source” to Tai et al. describes a beam-expanding light pipe that directs a line of light to a light panel for distribution over an area. Similarly, the luminaire arrangement described in U.S. Patent Application No. 2005/0231973 entitled “Efficient Luminaire with Directional Side-Light Extraction” by Cassarly et al. uses a light pipe with a light extraction structure for redirecting light along a backplane, such as for an exhibit or display case. As yet another example of this approach, U.S. Pat. No. 5,857,761 entitled “Illumination Device” to Abe et al. describes a light guide that spreads point source light into a light radiation plate.
Still other backlighting solutions employ flexible optical fibers for directing light from a single light source, then treated for spreading the light for emission behind an LCD panel. Different versions of this approach are described, for example, in U.S. Pat. No. 6,714,185 entitled “Back Lighting Apparatus of Liquid Crystal Display Using Optical Fiber” to Kim et al. and in U.S. Pat. No. 5,542,016 entitled “Optical Fiber Light Emitting Apparatus” to Kaschke.
As the above-cited examples attest, there has been considerable work directed to the goal of providing LED backlighting. However, although there have been a number of solutions proposed, there are significant drawbacks inherent to each type of solution, particularly when faced with the problem of backlighting for a display panel of standard laptop dimensions or larger. The 2-D matrix proposed in the '921 Deloy et al. disclosure would be difficult to implement inexpensively, of relatively high cost, bulky, and prone to uniformity problems. The light guide arrangement described in the '112 Kawai et al. disclosure is optimized for scanning applications that require a uniform line of light, rather than display backlighting applications. The molded panel arrangement described in the '224 DuNah et al. disclosure may work well enough for general illumination, but would be prone to uniformity problems for full-color display applications. This type of solution is increasingly expensive to manufacture in larger sizes and is subject to warping due to heat and mechanical stress. More importantly, such a solution does not provide good color mixing and would not be well suited to applications using solid-state light sources. Point source-to-panel configurations such as those described in the '3246 Parker application are impractical and exhibit uniformity problems for color and brightness for larger-sized displays. Light-guide-to-back-panel arrangements such as those described in the '661 Tai et al. disclosure are inefficient, are subject to poor uniformity, and are suitable only for relatively small displays. The use of treated optical fibers has advantages for small-scale handheld displays but would be impractical and inefficient for desktop or larger display designs.
In addition to these drawbacks, conventional solutions generally fail to address important challenges for high-quality color imaging, required for widespread commercialization and acceptance of LC displays. Color gamut is one important consideration that is of particular interest to display designers. Conventional CCFLs provide a measure of color quality that is acceptable for many applications, offering up to about 70% of the NTSC color gamut. Although this may be acceptable for laptop and computer monitor applications, it falls short of what is needed for full-color TV displays.
In contrast to CCFL light sources, LEDs and other solid-state light sources, because of their relatively high degree of spectral purity, are inherently capable of providing 100% or more of the NTSC color gamut. In order to provide this enlarged color gamut, three or more different-colored LEDs or other solid-state sources are needed. To support such an expanded color gamut when using LEDs and other solid-state light sources, a high level of color mixing is required from the backlighting apparatus. As is well known to those skilled in the imaging display art, achieving a good level of color uniformity when using solid-state light sources, such as Red (R), Green (G), and Blue (B) LEDs, is particularly challenging. Conventional backlighting solutions that employ larger-area light guides, such as those described above, would provide correspondingly inferior color mixing.
Other challenges related to backlighting for larger scale displays include the need for low-cost assembly, light efficiency, uniformity, and compact size. As noted earlier, conventional LED backlighting solutions fall short of what is needed to meet these additional requirements. Additionally, it would be particularly useful to eliminate the need for a reflective polarizer, which may be possible where uniformity and brightness are sufficiently improved.
Thus, it can be seen that there is a need for an LED backlight solution that can be inexpensively manufactured, has minimal thickness, and provides color mixing with good uniformity, high brightness, and high levels of efficiency.